1. Field of the Invention
This invention involves a,vertical-axis wind turbine (VAWT) whose blades are twisted so that their chords do not lie on radial planes.
2. Description of Related Art
Wind-driven rotating devices have been used for centuries, if not millennia, to convert the kinetic energy of the wind for use in such applications as pumping water and grinding meal. Especially during the last two decades, people in energy-hungry industrialized countries have been forced to face the practical implications of the concept "non-renewable" as it applies to fossil fuels such as coal and oil, on which most of these societies still rely.
Wind-driven turbines have proven to be among the more promising alternatives to fossil fuels. In a wind turbine, the momentum of the wind is converted into rotary energy, which is used to turn a generator shaft to create electric current. The two main types of wind turbines are the horizontal-axis wind turbine (HAWT) and the vertical axis wind turbine (VAWT).
Both HAWT and VAWT designs use either or both of two aerodynamic forces--drag and lift--to create torque on the generator shaft. For drag-based designs, the wind "pushes" the blades of the turbine around; these designs thus work on the same principle as a rotary wind-speed indicator, in which the wind blows "cups" attached to arms that extend from a vertical shaft. In lift-based designs, the blades typically have an airfoil shape, so that, like an airplane's wing or a sailboat's sail, it "flies" at an angle toward the wind.
In a horizontal-axis wind turbine, a propeller is mounted on a supporting structure such as a tower and rotates about a horizontal shaft, which is typically linked with the generator shaft via a gearbox. Since the direction of the wind will normally change, the propeller as a whole must be able to rotate about a vertical axis in order to face the wind and have the greatest possible efficiency. This creates problems of balance and wear on the bearings that allow the propeller to swivel around the vertical axis, especially since the generator is typically also mounted at the top of the supporting structure must rotate with the propeller. HAWT designs, with their horizontal axis for the propeller blades, therefore resemble the well-known windmill, not only as to structure, but also as to disadvantages.
In a vertical-axis wind turbine (VAWT), blades of the turbine are arranged substantially vertically, and they rotate about a vertical axis which is either also the axis of rotation of the generator shaft or is linked via a geartrain to the generator shaft. A major advantage of VAWT designs is that they do not require any re-orientation when the wind changes directions.
Examples of existing HAWT and VAWT designs are shown in the following texts:
U.S. Pat. No. 1,592,417 (Burke, Jul. 13, 1926); PA1 U.S. Pat. No. 1,835,018 (Darrieus, Dec. 8, 1931); PA1 U.S. Pat. No. 4,087,202 (Musgrove, May 2, 1978); PA1 U.S. Pat. No. 4,236,866 (Martinez, Dec. 2, 1980); PA1 U.S. Pat. No. 4,264,279 (Dereng, Apr. 28, 1981); PA1 U.S. Pat. No. 4,483,657 (Kaiser, Nov. 20, 1984); PA1 U.S. Pat. No. 4,561,826 (Taylor, Dec. 31, 1985); and PA1 U.S. Pat. No. 4,718,821 (Clancy, Jan. 12, 1988).
The primary goal of every wind turbine is to generate the maximum amount of torque to drive the generator shaft for a given wind speed. In other words, one wishes to maximize the efficiency of the wind turbine. At the same time, in order to achieve the goal of increasing the life-span of the turbine, it is important to minimize the stresses on the turbine structures, including the blades, the rotor, the various bearings, and especially on the geartrain, which is typically provided to convert the torque of the rotating blades to the proper RPMs for driving the generator shaft. As is so often the case, these two goals normally conflict.
In conventional VAWT designs, there are pronounced torque "peaks" at various angular positions for each blade. Additionally, each blade in conventional VAWT designs has several "null" positions, that is, angular positions at which it generates little or no torque at all. This makes the problem of torque load-unload cycles even worse. Each torque peak represents a peak load not only on the blades but also on the bearings and geartrain. Except for the turbine shown in the Darrieus patent, existing VAWT designs typically also include several other fixed or moving mounting or blade-adjustment structures, each of which is also subject to the non-uniform torque and are additional elements that can wear out and fail.
Torque peaks also reduce the efficiency of existing wind turbines, since energy that could be used to generate torque is dissipated as bending moment on the blades. Ideally, therefore, the torque profile for each blade should be as "smooth" as possible over the 360.degree. range of motion of the blade.
The problem of fatigue-induced failure is made worst by the stress-inducing loads being periodic. Even low-RPM turbines will typically be subjected to tens of thousands of load-unload cycles during the course of a day. The more torque peaks and "null" points are present over the 360.degree. rotation of each blade, the worse is the risk of fatigue failure.
Yet another disadvantage of many wind turbines is that they are not self-starting. As such, they require closer monitoring than if they were self-starting, or they require additional starting power and related equipment. The relatively high moment of inertia of windmills such as the Clancy device not only limits the achievable rotational velocity of the blades, but also reduces or precludes their ability to self-start.
It is accordingly an object of this invention to provide a self-starting vertical-axis wind turbine that has high efficiency and a low moment of inertia, yet has greater reliability than existing VAWT designs, with a blade configuration that allows it to generate torque more smoothly than existing designs while reducing the degree of wasteful bending moment on the blades and avoiding the need for additional, failure-prone struts, stabilizers, and linkages.